1. Field of the Invention
The present invention relates to a sheet conveying device, a sheet finisher, a sheet feeding device, an image forming apparatus, and a sheet conveying method.
2. Description of the Related Art
In recent years, finishers are in widespread use that is capable of correcting the posture or skew of a sheet, detecting and correcting a shift in a direction perpendicular to a sheet-conveying direction, and punching the sheet. Such finishers generally have any or all of functions of, for example, binding, sorting, saddle stitching, and center folding, in addition to punching.
A sheet having an image formed thereon has its leading edge abutting against an entrance roller of the finisher or a registration roller positioned downstream of the entrance roller for skew correction. Then, the position of an end face parallel to a sheet-conveying direction is detected to measure a shift of the sheet. The punching unit is then slid in a shifting direction by the amount of shift for punching. With this operation, accuracy of a punching hole position is improved, thereby improving accuracy of punching-hole alignment for a plurality of sheets.
For example, Japanese Patent Application Laid-open Publication No. 2003-212424 discloses a conventional technology related to such a finisher. In the conventional technology, an entrance roller serves as a registration roller. While a sheet abuts on the entrance roller to be corrected on its posture or skew, a delivery roller of an image forming apparatus that delivers the sheet continues to be driven. Therefore, while the posture of the sheet is corrected, the sheet is deformed (e.g., curls or becomes wavy) between the entrance roller of the finisher and the delivery roller on the image forming apparatus side. Although skew correction is performed with this curl, if a linear velocity of the sheet delivered from the image forming apparatus is increased, larger curl is formed.
That is, the conventional technology can be applied to a low or intermediate-speed image forming apparatus; however, if it is applied to a high-speed image forming apparatus, a sheet is deformed to the extent that it difficult to correct skew with accuracy and to stably convey the sheet.